The present invention relates to a surface-mountable fuse having an outer insulative housing. More particularly, the present invention relates to a fuse having an outer housing made from a material which improves the overall current rating of the fuse.
Fuses are used to conduct current under normal conditions and to break a circuit under overload conditions. Many electrical fuses, including the fuses employed in automotive vehicles, comprise a pair of generally terminal leads which are electrically connected to one another. The electrical connection between the terminal leads of the fuse is selected in accordance with the specified current to be carried by the circuit into which the fuse is incorporated. An electrical current level which exceeds the specified levelxe2x80x94the overload conditionxe2x80x94will damage the electrical connection between the terminal leads of the fuse, thereby breaking the circuit and preventing more serious damage to other electrical components.
In some fuses, an insulative housing is used to contain heat and provide for quick melting of the fusible elements under overload conditions. The most suitable and most widely used materials for the housings of electric fuses are flame-retardant polymers, ceramic materials, and synthetic-glass-cloth laminates. It is common for these materials to contain a halogenated material for increased flame retardation. However, under high-voltage conditions when a great deal of heat is generated within the fusible elements of the fuse, prior art housings vaporize to form an ion plasmaxe2x80x94a process called ablating. This process involves arcing within the resultant plasma, which is further increased by the presence of halogen ions (from the flame retardant material).
In prior art fuses, the specific current rating of any fuse (the threshold current beyond which the fusible connection will break) is substantially determined by the amperage capacity of the fusible connection. In other words, to increase the rating of the fuse, one would increase the amperage capacity of the fusible connection. The present invention breaks from this convention and provides for a higher rated fuse by constructing the insulative housing from a specific material.
By providing a low-voltage fuse with a housing made from a material which (1) burns cleanly, (2) has greater ablative qualities over other fuse housing materials, and (3) resists ignition at high-voltage energies, a higher-voltage electric fuse is produced without increasing the amperage capacity of the fusible connection.
The present invention discloses an electric fuse having an insulative housing, two terminal leads extending from the housing, and a fusible link electrically connecting the two terminal leads. The insulative housing is made from a material that increases the overall rating of the fuse. In accordance with the increased current rating, the housing of the present fuse electrically insulates the fusible link and is preferably made from a material which burns cleanly and has improved ablative qualities. The housing material, therefore, produces little or no carbon tracking, and any plasma of the housing material generated in high energy periods does not accelerate arcing. It is preferred that the housing material should be free of halogens, which when ionized accelerate arcing within a plasma. Preferred insulative materials (e.g., nylon and PET polyester) increase the current rating of fuses according to the present invention by as much as ten times the current rating of electrical fuses having a conventional polysulfone housing.
In one embodiment of the present invention, the material used for the fuse housing has an arc resistance within the range of about 60 to about 120 seconds. The housing may be made from a material having a comparative tracking index within the range of about 250 to about 400 volts. The housing material used may also have a high-amperage arc ignition resistance of greater than 120 arcs. Additionally, the material may have a high-voltage arc tracking rate within the range of about 25 to about 80 mm/minute, and a hot wire ignition value of greater than 120 seconds. It is preferred that the material used for the fuse housing should meet or exceed each of these target electrical and flammability values.